This invention pertains to the field of mode-locked lasers and more specifically to the field of servo-controlled mode-locked lasers.
In any passively mode-locked laser, whether achieved by the use of an intracavity modulator or by synchronous pumping, it is necessary to match the laser cavity frequency, C/2L, to the driving frequency with great precision in order to achieve the best mode locking (C is the speed of light and L is the laser cavity length). In some mode-locked lasers, such as Nd:YAG, the major part of the cavity beam is open to the atmosphere. Since n-1.congruent.3.times.10.sup.-4 for air at standard temperature and pressure (S.T.P.), where n is the index of refraction, and since daily barometric pressure variations can be as great as 0.1 ATM, fractional changes in C/2L as great as 30.times.10.sup.-6 are to be expected.
One general approach to achieving stability is to detect the difference in relative timing, or phase, between the mode-locked laser output pulses and the driving mechanism. This phase-shift measurement has been accomplished in the prior art by first converting a sample of the laser pulses into an electrical analog and then making the phase comparison in an electrical circuit. This approach has been successful, especially in those lasers where the mode-locking is achieved by way of an electrically driven modulator. However, this approach has several limitations. Among these are the following: (1) lack of a detector with sufficiently fast response to truly represent picosecond pulses in its electrical output, (2) difficulties in maintaining sufficient phase stability in the associated r.f. circuitry, (3) possibility of drift between the phase of the r.f. drive voltage and the phase of the modulator response, and (4) the need to provide for and to discover empirically the phase shift of the r.f. required for zero output of the phase-sensitive detector at the point of optimium adjustment of the laser cavity length.